Role of PTEN in Mitosis and Chromosome Stability Project Summary The chromatin environment shapes DNA-based processes and recent studies reveal that the PTEN tumor suppressor con- trols both DNA duplication/segregation and chromatin remodeling. A fundamental question is how PTEN mediates the interplay between the genome and the epigenome to ensure faithful transmission of genetic materials. Our long-term goal is to illuminate the function of PTEN in maintaining mitotic chromosome stability and to understand why this function is a major driving force in tumor suppression. The overall objective of this project is to identify critical regulatory elements in both mitosis-intrinsic machinery and global chromatin architecture that work together under the control of PTEN to promote mitotic fidelity. In support of this goal, our recent studies show that depletion of PTEN results in spindle shorten- ing and pole fragmentation, accompanied by misalignment and non-disjunction of chromosomes, leading to catastrophic mitotic failure and polyploidy. These observations suggest that PTEN is directly involved in mediating the interaction be- tween chromosomes and the mitotic spindle. In addition, metaphase cells lacking PTEN exhibit prominent chromosome entanglement, centromere breakage, and acentric anaphase bridges, indicating structural chromosome instability likely due to impaired chromatin architecture. Interestingly, our published data demonstrate that PTEN regulates histone anchor- age and modification on chromatin and thereby affects the chromatin compaction status. These results collectively support our hypothesis that PTEN maintains structural and numerical chromosome stability by coordinating mitotic chromatin compaction and mitotic spindle assembly for proper chromosome alignment and segregation. We will test this hypothesis by pursuing two specific aims. In the first aim, we will demonstrate impairment of mitotic chromatin compaction and epi- genetic deregulation of kinetochore assembly as a prevailing phenotypic consequence of PTEN dysfunction. High- resolution imaging and biochemical approaches, in combination with knock-in animal models developed in our laborato- ry, will be used to analyze PTEN function in promoting mitotic chromatin condensation. In the second aim, we will delin- eate the mechanism underlying the functional interaction among interrelated PTEN pathways in governing spindle activity and chromosome behavior. Using a comprehensive set of cellular and molecular biology approaches, coupled with newly generated transgenic mouse strains, we will depict a PTEN-associated mitotic signaling network comprised of both struc- tural and functional regulatory elements that act in concert to guard the genome. Successful completion of these aims will fundamentally advance our mechanistic understanding of the multifaceted function of PTEN in mitosis and chromosome stability. Data from our studies will illustrate how PTEN deficiency triggers a deteriorative interaction between deregulat- ed mitotic machinery and unfavorable chromatin milieus, leading to structural and numerical chromosome aberrations. Newly discovered mitotic pathways and targets in this innovative project will contribute to precision medicine through tailoring targeted therapies based on the genetic and epigenetic signatures of each cancer patient.